Method and apparatus for the production of metal hydrides



Nov. 9, 1965 w. B. WHITNEY 3,216,914

METHOD AND APPARATUS FOR THE PRODUCTION OF METAL HYDRIDES Filed May 27, 1963 MIXING &

REACTION MEANS METAL HYDRIDE MERCURY RECYCLE MERCURY RECYCLE INVENTOR.

W. B. WHITNEY A TTORNEVS United States Patent 3,216,914 METHOD AND APPARATUS FOR THE PRODUCTION OF METAL HYDRIDES William B. Whitney, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Filed May 27, 1963, Ser. No. 283,435 9 Claims. (Cl. 204S6) This invention relates to a method and an apparatus for the production of metal hydrides.

Metal hydrides are useful sources of hydrogen. As such they are frequently used as a source of hydrogen to be used for any purpose, including hydrogenation processes. Metal hydrides are also good reducing agents and good drying agents. Some metal hydrides, such as sodium hydride, are also useful as catalysts in certain catalytic processes. Metal hydrides are thus a valuable class of chemical compounds of many and varied uses. The development of the known uses, and also of new uses, for metal hydrides has been hampered by inadequate methods of preparation.

One common method of producing metal hydrides is by the reaction of gaseous hydrogen with the tree metal. One difiiculty encountered in this method is obtaining the metal in suitable form. For example, sodium hydride is made commercially by the reaction of hydrogen on molten sodium metal. A big difficulty in this process is the production of the metallic sodium in an efficient and economical manner. Metallic sodium is produced commercially by the electrolysis of molten sodium chloride and also by electrolysis of molten sodium hydroxide. These processes are expensive and are destructive to the apparatus employed due to the high temperatures involved. Other difiiculties in the commercial processes are recognized and for which there have been no satisfactory solution. Some metal hydrides can be prepared by chemical means. However, such processes involve difliculties such as purity of reagents, control of reaction conditions, etc.

I have discovered a method of preparing metal hydrides which overcomes the above-described difiiculties. I have discovered that metal hydrides can be produced by mixing together an amalgam of a metal and an amalgam of ammonium and separating the metal hydride which forms. Thus, broadly speaking, the present invention resides in a process for the preparation of a metal hydride, which process comprises mixing an amalgam of said metal with an ammonium amalgam, and recovering the metal hydride formed. In another broad aspect, the invention resides in a combination of apparatus for carrying out said process.

Thus, an object of this invention is to provide a new method for the production of metal hydrides. Another object of this invention is to provide a combination of apparatus which can be employed to produce metal hydrides. Other aspects, objects and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Amalgams of mercury with numerous metals are well known. These amalgams are commonly formed by subjecting an electrolyte solution containing the desired metal ion to electrolysis in an electrolytic cell provided with a liquid mercury cathode. Amalgams of mercury with ammonium can also be prepared in like manner by using an electrolyte containing the desired ammonium ion. Aqueous solutions of the corresponding salts or hydroxides are commonly used as the electrolyte solutions. The metal or ammonium discharged at the cathode is absorbed and/or amalgamated in the mercury of the mercury cathode, thus forming a metal amalgam or an ammonium amalgam. I-n continuous processes wherein the electrolyte is continuously replenished, said amalgam can be drawn oft continuously as they are formed.

Amalgams prepared in electrolytic cells commonly contain from about 0.1 to about 1 percent by weight of the amalgamated material. The amount of the amalgamated material in the amalgam will depend upon the operating conditions employed in the electrolytic cell. However, as the concentration of the amalgamated material in the amalgam increases, the efiiciency of the cell decreases. Said cells can be operated to produce higher concentrations of amalgamated material in the amalgam but when so operated the efficiency materially decreases and there is a tendency for the amalgam to solidify. Insofar as this invention is concerned, when the amalgams used in the practice of the invention are prepared in electrolytic cells, said cells can be operated under any suitable conditions and the invention is not limited to any particular conditions for the operation of said cells. Operating conditions for electrolystic cells used in the production of amalgams are well known to those skilled in the art. For example, see US. Patent 2,829,950, issued April 8, 1958, to G. L. Cunningham.

When the concentration of amalgamated material in an amalgam is expressed in terms of weight percent, the values appear to be low. Actually, however, the mol fraction value is much higher when one considers the differences in molecular weight of the amalgamated material and the mercury in the amalgam. For example, a 1 weight percent sodium amalgam contains about 8 mol percent of sodium and a 1 weight percent of lithium amalgam contains about 20 mol percent of lithium.

Amalgams of any suitable concentration, including concentrations normally produced by electrolytic cells, can be employed in the practice of the invention. It is also within the scope of the invention to concentrate said electrolytically produced amalgams, such as by distillation of mercury therefrom. Methods for concentrating such amalgams are well known to those skilled in the art. However, the concentration of the amalgamated material in the amalgams used in the practice of the invention should be such that at least one of said amalgams is liquid, or at least essentially so, at the temperatures employed in the mixing of said amalgams. The metals employed in the practice of the invention are usually the alkali metals such as sodium, potassium, lithium, rubidium, and cesium and the alkaline earth metals such as calcium, barium, and strontium. Hydrides of these metals are all well known and are the ones for which the most uses have been developed. However, other metals such as aluminum and magnesium can also be used. Thus, the only actual limitation on the metals which can be used in the practice of the invention is that the metal will form both an amalgam and a hydride.

The prefered source of ammonium in the ammonium amalgams used in the practice of the invention is ammonium hydroxide or an ammonium salt such as ammonium chloride, ammonium ulfate, ammonium nitrate and other water soluible salts. However, lower alkyl substituted ammonium compounds wherein the alkyl groups contains from 1 to 4 carbon atoms can also be used in the practice of the invention. Examples of such compounds include, among others, methyl ammonium hydroxide, and ethyl ammonium hydroxide, and salts of methylamine, ethylamine, and propylam-ine. Thus, herein and in the claims, unless otherwise specified, the term an ammonium amalgam is employed generically and refers to and includes ammonium amalgams such as result when a solution of ammonium hydroxide or a solution of a salt such as ammonium chloride is used as the electrolyte, and also includes lower alkyl substituted ammonium amalgams such as result when a solution of a lower alkyl substituted ammonium compound such as methyl ammonium hydroxide or methyl ammonium chloride is used as the electrolyte.

Referring now to the drawing, the invention will be more fully explained. Said drawing is a diagrammatic flow sheet of one process and a schematic representation of one combination of apparatus which can be employed for the production of metal hydrides in accordance with the invention.

In said drawing there are shown a first electrolytic cell and a second electrolytic cell 10. Each of said cells is provided with a liquid mercury cathode means designated 11 and 11', respectively. Said cathode means can be any suitable form of liquid mercury cathode known to the art. Each of said cells is also provided with a suitable anode means designated 12 and 12', respectively. Said anodes can be any suitable type of anode known to the art. Materials commonly used for said anodes include graphite and platinum. Said cathode and said anode in each of said cells are connected to a suitable source of direct current in conventional manner by means of the lead wires 13, 13, 14 and 14' shown. Said first and second cells are provided with suitable conduit means 16 and 16, respectively, for introducing and replenishing the electrolyte solution therein. Said cells are also provided with suitable conduit means 17 and 17' for removing materials such as oxygen or chlorine which are liberated at the anode. Some hydrogen will be liberated at the cathode and this can also be removed through conduits 17 and 17' or, if desired, other suitable means can be provided for removing the hydrogen separately. If desired, means can be provided for recovering said materials which are liberated at the anode or cathode.

Means are provided within each of said cells for separating the amalgam formed at the cathode from the mercury comprising said cathode. As illustrated, said separating means comprises a weir 18 and 18 which defines one wall or boundary of the mercury cathode. When the cell is operated in a continuous manner, the amalgam formed at the cathode is caused to overflow said weir by the make-up mercury entering the cathode chamber and collects as a pool of amalgam 19 and 19' in one compartment of the cell. Said weirs 18 and 18' can be fab ricated from a suitable plastic or other non-conducting material as shown. It will be understood that the walls of said cells are also fabricated of a suitable non-conducting material. Said cells can each be provided with suitable heating and cooling means, such as a coil immersed in the electrolyte through which heating medium or refrigerant can be passed, for controlling the tempera ture within the cell.

A first conduit means 21 and a second conduit means 21 are connected to said first and second cells, respectively, for withdrawing amalgam therefrom and passing same to a mixing and reaction vessel 22. In the laboratory said amalgams can be conveniently mixed employing a mortar and pestle or other siutable similar mixing apparatus. In larger scale operations any suitable mixing apparatus wherein the amalgams can be intimately mixed and reacted can be employed. Stirred reaction vessels of many types, for example, are well known in the art. Suitable cooling means 23 and 23' are disposed in said first and second conduit means 21 and 21. Said cooling means can be any suitable type of cooler such as an indirect heat exchanger wherein the amalgams can be cooled by heat exchange with any suitable cooling medium.

In many instances, since the amalgams are preferably withdrawn from the bottom of the pool of amalgam which collects in the electrolyte cell, the amalgams will be sufficiently dry to permit direct passage through conduit means 21 and 21' and coolers 23 and 23' disposed therein to said reaction vessel 22. However, if desired or necessary, suitable drying means 24 and 24 can also be disposed in said conduit means 21 and 21 by means of the valve and bypass arrangements shown. Said drying means can comprise any suitable means for removing moisture from the amalgams. For example, said drying means can comprise a vessel or chamber containing cotton linters, suitably formed absorbent paper, bauxite, alumina, or other materials which will absorb moisture from the amalgam. Although not illustrated in the drawing, it will be understood to be within the scope of the invention to employ a plurality of said dryers connected in series or in parallel, if desired.

A third conduit 26 is connected to said reaction vessel 22 for withdrawing reaction mixture therefrom and passing same to a separator 27. Said separator 27 can comprise any suitable means for separating the solid metal hydride from the liquid mercury. As here shown, said separation means comprises a filtering vessel provided with an inclined screen 28 therein. Since the metal hydride will float on top of the mercury liberated by the reaction between the amalgams, said separating means 27 can comprise a suitable compartmented vessel provided with suitable weirs and battles for skimming the metal hydride from the top of the mercury, withdrawing the metal hydride from an upper portion of the vessel, and withdrawing the separated mercury from the bottom of the vessel. A fourth conduit means 29 extends from an upper portion of said separating means 27 to said second electrolytic cell 10' for the recycle of liberated ammonia or alkyl substituted ammonia to said cell. A fifth conduit means 31 is provided for withdrawing separated metal hydride from said vessel 27. A sixth conduit means 32 and a seventh conduit means 32' are connected to the lower portion of said separation vessel 27 and extend respectively to the cathode compartments 13 and 13 of said first and second cells 10 and 10' for returning separated mercury to said cathodes. Conduits 33 and 33' are provided for introducing make-up mercury as required.

In the operation of the above-described apparatus, suitable electrolyte solutions are placed in each of said cells 10 and 10' by means of conduits 16 and 16, respectively. A presently preferred electrolyte for use in the metal electrolysis cell or first electrolytic cell 10 is the hydroxide of the desired metal, such as potassium hydroxide. Similarly, a presently preferred electrolyte for use in the nitrogen base electrolysis cell or second electrolytic cell 10' is ammonium hydroxide. Electrolytes of any suitable concentration can be employed in the practice of the invention. The choice of the actual concentration used will depend upon the particular electrolyte, the temperature, type and size of cell, and other factors, all known to those skilled in the art. It is frequently preferred that the electrolyte solutions be substantially saturated solutions although less concentrated solutions are often used. The metal electrolysis cell or first electrolytic cell 10 is commonly operated at temperatures ranging from about room temperature up to about C. However, for reasons discussed further hereinafter, the nitrogen base electrolysis cell or second electrolytic cell 10 is preferably operated at lower temperatures such as in the neighborhood of 0 to 30 C.

After the cells have been charged, current is applied thereto in conventional manner and the amlagam forms at the cathode. When operating in a continuous manner, said amalgam, being less dense than mercury, fioats on the mercury and overflows into the compartment as shown and is withdrawn via conduits 21 and 21' and passed, either with or without drying as desired or necessary, to the reactor 22 wherein they are mixed with the resulting formation of the metal hydride. While it is not intended to limit the invention by the following explanation, it is presently believed that the reaction which takes place upon the mixing of a metal amalgam, such as potassium amalgam, and an ammonium amalgam, such as the amalgam ,5. obtained by the electrolysis of ammonium hydroxide, can be represented by the Equation Since the ammonium amalgams are unstable at temperatures above about 30 C., the mixing of the amalgams in reactor 22 should be carried out at temperatures below about 30 C., preferably below about 10 C. However, the temperature of the mixing should be high enough so that at least one of the amalgams, and preferably both, is liquid, or essentially liquid. Said mixing temperature will usually be at least 35 C. and preferably at least 30 C. Thus, the temperature of the mixing can range from about -35 C. to about +30 C. As will be understood by those skilled in the art, the composition of the amalgams as well as the temperature will determine whether or not the amalgam is liquid or solid. Amalgams containing high contents of amalgamated material ordinarily freeze above the freezing point of mercury. Thus, in choosing mixing temperatures for the particular amalgams to be mixed, the temperature, the particular amalgamated material, and the concentration of the amalgamated material in the amalgam must be taken into consideration.

The pressure employed during the mixing of the amalgams is not critical and is preferably about atmospheric or sufiiciently above atmospheric for convenience in operations. For example, pressures slightly above atmospheric aid in the return of the liberated ammonia or alkyl substituted ammonia to the second electrolytic cell 10'. However, it is within the scope of the invention to operate the separating means 27 at pressures lower than atmospheric to aid in removing the liberated ammonia or alkyl substituted ammonia and return of same to second electrolytic cell 10'. In general, greatly elevated or reduced pressures are of no benefit in the mixing and reaction of the amalgams for the formation of the metal hydrides.

In mixing the amalgams an excess of the ammonium amalgam or alkyl substituted ammonium amalgam over that required stoichiometrically is normally employed. This tends to drive the reaction to completion and allows for some decomposition of said ammonium amalgams without formation of the metal hydride, such as the decomposition of ammonium amalgam to ammonia and hydrogen.

The reaction in which the metal hydride is formed is quite rapid. Therefore, relatively small mixing and reaction zones can be employed. Reaction times in the order of about 1 to 60 minutes, usually 2 to 20 minutes, can be employed in the practice of the invention.

The following example will serve to further illustrate the invention.

Example A potassium amalgam was prepared by electrolysis at room temperature of an approximately 30 Weight percent aqueous potassium hydroxide solution in an electrolytic cell provided with a liquid mercury cathode. An ammonium amalgam was prepared similarly using a concentrated (0.9 sp. gr.) aqueous ammonium hydroxide solution. The amalgams were then separated from the aqueous solutions and dried. The potassium amalgam was liquid. The ammonium amalgam had a soft, buttery consistency.

Portions of each of the amalgams were cooled to approximately C. and mixed together. Said amalgams were mixed in approximately equal volume portions. A hard, gritty solid formed rapidly and floated on top of the mercury. The mixture was allowed to warm up, and the hard solid was separated by straining using a IOU-mesh nickel screen.

A portion of the separated hard solid was dropped into water. A very rapid reaction evolving gas started at once but stopped very quickly and substantially completely after a short time. This is characteristic of potassium hydride.

A portion of the potassium amalgam was dropped into water. Gas was evolved vigorously at first, .but gradually slowed down and continued for more than an hour. This is characteristic of potassium amalgam and quite different from the reaction of potassium hydride, showing that potassium hydride had been prepared by the reaction between the two amalgams.

While the invention has been described above in terms of amalgams produced by electrolytic means, the invention is not limited to amalgams so produced. Amalgams produced in any manner can be used in the practice of the invention. For example, many metal amalgams can he easily prepared by merely mixing the free metal in suitable form with mercury.

While certain embodiments of the invention have been described for illustrative purposes, the invention obviously is not limited thereto. Various other modifications will be apparent to those skilled in the art in view of this disclosure. Such modifications are within the spirit and scope of the invention.

I claim:

1. A process for the preparation of a metal hydride, which process comprises mixing an amalgam of said metal with an ammonium amalgam, "and recovering the metal hydride formed.

2. A process for the preparation of a metal hydride, which process comprises: reacting an amalgam of said metal with an ammonium amalgam and forming a reaction mixture containing said metal hydride; and recovering said metal hydride from said reaction mixture.

3. A process for the preparation of a metal hydride, which process comprises: mixing an amalgam of said metal with an ammonium amalgam at a temperature of not more than about 30 C., but which is sufficient to cause at least one of said amalgams to be essentially liquid, and thus forming a reaction mixture containing said metal hydride; and recovering said metal hydride from said reaction mixture.

4. The process of claim 3 wherein said temperature is within the range of about -35 C. to about +30 C.

5. The process of claim 4 wherein said metal amalgam is potassium amalgam, said an ammonium amalgam is ammonium amalgam, and said metal hydride is potassium hydride.

6. A process for the production of a metal hydride, which process comprises, in combination, the steps of: subjecting a first electrolyte solution containing metal ions to electrolysis in a first electrolytic cell provided with a first liquid mercury cathode, whereby the metal liberated at said first cathode is amalgamated with mercury forming an amalgam of said metal; subjecting a second electrolyte solution containing cations selected from the group consisting of ammonium ion and alkyl substituted ammonium ion to electrolysis in a second electrolytic cell provided with a second liquid mercury cathode, whereby the said cations which are liberated at said second cathode are amalgamated with mercury forming an ammonium amalgam; removing said metal amalgam =frorn said first electrolytic cell; removing said an ammonium amalgam from said second electrolytic cell; mixing said metal amalgam with said an ammonium amalgam to form a reaction mixture containing said metal hydride and liberated mercury; and recovering said metal hydride from said reaction mixture.

7. A process for the production of a metal hydride, which process comprises, in combination, the steps of: subjecting a first electrolyte solution containing metal ions to electrolysis in a first electrolytic cell provided with a first liquid mercury cathode, whereby the metal liberated at said first cathode is amalgamated with mercury forming an amalgam of said metal; subjecting a second electrolyte solution containing cations selected from the group consisting of ammonium ion and alkyl substituted ammonium ion to electrolysis in a second electrolytic cell provided with a second liquid mercury cathode, whereby the said cations which are liberated at said second cathode are amalgamated with mercury forming an ammonium amalgam; removing said metal amalgam from said first electrolytic cell; removing said an ammonium amalgam from said second electrolytic cell; mixing said metal amalgam with said an ammonium amalgam to form a reaction mixture containing said metal hydride and mercury; recovering said metal hydride from said reaction mixture; and returning at least a portion of said mercury to at least one of said electrolytic cells.

8. A process according to claim 7 wherein said first electrolyte is potassium hydroxide, said second electrolyte is ammonium hydroxide, and said metal hydride is potassium hydride.

9. A continuous process for the production of a metal hydride, which process comprises, in combination, the steps of: subjecting a first electrolyte solution containing metal ions to electrolysis in a first electrolytic cell provided with a first liquid mercury cathode, whereby the metal liberated at said cathode is amalgamated with mercury forming an amalgam of said metal; subjecting a second electrolyte solution containing cations selected from the group consisting of ammonium ion and alkyl substituted ammonium ion to electrolysis in a second electrolytic cell provided with a second liquid mercury cathode, whereby the said cations which are liberated at said second cathode are amalgamated with mercury forming an ammonium amalgam; removing said metal amalgam from said first electrolytic cell; removing said an ammonium amalgam from said second electrolytic cell; drying each of said removed amalgams; cooling each. of said dried amalgams to a temperature within the range of from 35 to +30 C.; mixing said cooled metal amalgam with said cooled an ammonium amalgam to form a reaction mixture containing said metal hydride and mercury; recovering said metal hydride from said reaction mixture; and returning separate portions of said mercury to each of said electrolytic cells.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES J. Chem. Soc., 1951, 1731-6.

JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner. 

1. A PROCESS FOR THE PREPARATION OF A METAL HYDRIDE, WHICH PROCESS COMPRISES MIXING AN AMALGAM OF SAID METAL WITH AN AMMONIUM AMALGAM, AND RECOVERING THE METAL HYDRIDE FORMED. 